An on-line information system typically includes one or more computer systems (the servers) that makes information available so that other computer systems (the clients) can access the information. Each server manages access to the information, which can be structured as a set of independent on-line services. A server and client communicate via messages conforming to a communication protocol and sent over a communication channel such as a computer network or through a dial-up connection.
Typical uses for on-line services include document viewing, electronic commerce, directory lookup, on-line classified advertisements, reference services, electronic bulletin boards, document retrieval, electronic publishing, keyword searching of documents, technical support for products, and directories of on-line services. The on-line service may make the information available free of charge, or for a fee, and may be on publicly accessible or private computer systems.
Information sources managed by the server may include files, databases, and applications on the server system or on an external computer system. The information that the server provides may simply be stored on the server, may be converted from other formats manually or automatically, may be computed on the server in response to a client request, may be derived from data and applications on the server or other machines, or may be derived by any combination of these techniques.
The user of an on-line service typically uses a specialized computer program, such as a browser, that is executed on the client system to access the information managed by an on-line service. Possible user capabilities include viewing, searching, downloading, printing, editing, and filing the information managed by the server. The user may also price, purchase, rent, or reserve services or goods offered through the on-line service.
An exemplary on-line service for catalog shopping might work as follows. A user running a program on a client system requests a connection to the catalog shopping service using a service name that either is well known or can be found in a directory. The request is received by the server employed by the catalog shopping service, and the server returns an introductory document that asks for an identifier and password. The client program displays this document, the user fills in an identifier and password that were assigned by the service in a previous visit, and the information is sent to the server. The server verifies the identifier and password against an authorization database, and returns a menu document that is then presented to the user. Each time the user selects a menu item, the selection is sent to the server, and the server responds with the appropriate new page of information, possibly including item descriptions or prices that are retrieved from a catalog database. By selecting a series of menu items, the user navigates to the desired item in the catalog and requests that the item be ordered. The server receives the order request, and returns a form to be completed by the user to provide information about shipping and billing. The user response is returned to the server, and the server enters the order information into an order database.
On-line services are available on the World Wide Web (WWW), which operates over the global Internet. The Internet is a publicly accessible wide area network (WAN) comprising a multitude of generally unrelated computer networks that are interconnected. Similar services are available on private networks called “Intranets” that may not be connected to the Internet, and through local area networks (LANs). The WWW and similar private architectures provide a “web” of interconnected document objects. On the WWW, these document objects are located at various sites on the global Internet. A more complete description of the WWW is provided in “The World-Wide Web,” by T. Berners-Lee, R. Cailliau, A. Luotonen, H. F. Nielsen, and A. Secret, Communications of the ACM, 37 (8), pp. 76-82, August 1994, and in “World Wide Web: The Information Universe,” by Berners-Lee, T., et al., in Electronic Networking: Research, Applications and Policy, Vol. 1, No. 2, Meckler, Westport, Conn., Spring 1992.
Among the types of document objects in an on-line service are documents and scripts. Documents that are published on the WWW are written in the Hypertext Markup Language (HTML). This language is described in HyperText Markup Language Specification-2.0, by T. Berners-Lee and D. Connolly, RFC 1866, proposed standard, November 1995, and in “World Wide Web & HTML,” by Douglas C. McArthur, in Dr. Dobbs Journal, December 1994, pp. 18-20, 22, 24, 26 and 86. Many companies also are developing their own enhancements to HTML. HTML documents are generally static, that is, their contents do not change over time unless modified by a service or web site developer. HTML documents can be created using programs specifically designed for that purpose, such as Microsoft Corporation's FRONTPAGE™ Web Page publishing program, by editing a text file, or by executing a script file.
The HTML language is used for writing hypertext documents, which are more formally referred to as Standard Generalized Markup Language (SGML) documents that conform to a particular Document Type Definition (DTD). An HTML document includes a hierarchical set of markup elements; most elements have a start tag, followed by content, followed by an end tag. The content is a combination of text and nested markup elements. Tags, which are enclosed in angle brackets (‘<’ and ‘>’), indicate how the document is structured and how to display the document, as well as destinations and labels for hypertext links. There are tags for markup elements such as titles and headers, text attributes such as bold and italic, lists, paragraph boundaries, links to other documents or other parts of the same document, in-line graphic images, and for many other features.
The following lines of HTML briefly illustrate how the language is used:                Some words are <B>bold</B>, others are <I>italic</I>. Here we start a new paragraph.<P>Here's a link to        the <A HREF=“http://www.microsoft.com”>Microsoft Corporation </A>homepage.        
This sample document is a hypertext document because it contains a hypertext “link” (hyperlink) to another document, in the line that includes “HREF=.” The format of this link is described below. A hypertext document may also have a link to other parts of the same document. Linked documents may generally be located anywhere on the Internet. When a user is viewing the document using a client program called a Web browser (described below), the links are displayed as highlighted words or phrases. For example, using a Web browser, the sample document above might be displayed on the user's screen as follows:                Some words are bold, others are italic. Here we start a new paragraph.        Here's a link to Microsoft Corporation homepage.        
In the Web browser, the link may be selected, for example, by clicking on the highlighted area with a mouse. Typically, the screen cursor noticeably changes (shape and/or color) when positioned on a hypertext link. Selecting a link will cause the associated document to be displayed. Thus, clicking on the highlighted text “Microsoft Corporation” would fetch and display the associated homepage for that entity.
The HTML language also provides a mechanism (the image or “IMG” element) enabling an HTML document to include an image that is stored as a separate file. When the end user views the HTML document with a browser program, the included image is displayed to the user as part of the document, at the point where the image element occurred in the document.
Another kind of document object in a web is a script. A script is an executable program or a set of commands stored in a file that can be run by a server program called a Web server (described below) to produce an HTML document that is then returned to the Web browser. Typical script actions include running library routines or other applications to fetch information from a file or a database, or initiating a request to obtain information from another machine, or retrieving a document corresponding to a selected hypertext link. A script may be run on the Web server when, for example, the end user selects a particular hypertext link in the Web browser, or submits an HTML form request. Scripts are usually written by a service developer in an interpreted language such as Basic, Practical Extraction and Report Language (Perl), or Tool Control Language (Tcl) or one of the Unix operating system shell languages, but they also may be written in more complex programming languages such as “C” and then compiled to produce an executable program. Programming in Tcl is described in more detail in Tcl and the Tk Toolkit, by John K. Ousterhout, Addison-Wesley, Reading, Mass., USA, 1994. Perl is described in more detail in Programming in Perl, by Larry Wall and Randal L. Schwartz, O'Reilly & Associates, Inc., Sebastopol, Calif., USA, 1992.
Each document object in a web has an identifier called a Universal Resource Identifier (URI). These identifiers are described in more detail in T. Berners-Lee, “Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web,” RFC 1630, CERN, June 1994; and T. Berners-Lee, L. Masinter, and M. McCahill, “Uniform Resource Locators (URL),” RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994. A URI allows any object on the Internet to be referred to by name or address, such as in a link in an HTML document as shown above. There are two types of URIs: Universal Resource Name (URN) and Uniform Resource Locator (URL). A URN references an object by name within a given name space. The Internet community has not yet defined the syntax of URNs. A URL references an object by defining an access algorithm using network protocols. An example of a URL is “http://www.microsoft.com”. A URL has the syntax “scheme://host:port/path?search” where                “scheme” identifies the access protocol (such as HTTP, FTP, or GOPHER);        “host” is the Internet domain name of the machine that supports the protocol, and comprises the fully qualified domain name of a network host, or its IP address as a set of four decimal digit groups separated by “.”. Fully qualified domain names take the form of a sequence of domain labels separated by “.”, each domain label starting and ending with an alphanumerical character and possibly also containing “-” characters. The rightmost domain label will never start with a digit, though, which syntactically distinguishes all domain names from the IP addresses (See Section 3.5 of RFC 1034 and Section 2.1 of RFC 1123).        “port” is the transmission control protocol (TCP) port number of the appropriate server (if different from the default);        “path” is a scheme-specific identification of the object. It supplies the details of how the specified resource can be accessed. Note that the “/” between the host (or port) and the path is NOT part of the path; and        “search” contains optional parameters for querying the content of the object.        
URLs are also used by web servers and browsers on private computer systems, Intranets, or networks, and not just for the WWW.
The HTTP URL scheme is used to designate Internet resources that may be accessed using HTTP. The HTTP URL has the syntax “http://<host>:<port>/<path>?<searchpart>”, where <host> and <port> are as described above. If :<port> is omitted, the port defaults to 80. No user name or password is allowed. <path> is an HTTP selector, and <searchpart> is a query string. The <path> is optional, as is the <searchpart> and its preceding “?”. If neither <path> nor <searchpart> is present, the “/” may also be omitted. Within the <path> and <searchpart> components, “/”, “;”, “?” are reserved. The “/” character may be used within HTTP to designate a hierarchical structure.
There are generally two types of URLs that may be used in the hypertext link: absolute URLs and relative URLs. An absolute URL includes a protocol identifier, a machine name, and an optional HTTP port number. A relative URL does not include a protocol identifier, machine name or port, and must be interpreted relative to some known absolute URL called the base URL. The base URL is used to determine the protocol identifier, machine name, optional port, and base directory for a relative URL. For further discussion of URL format and usage, see the document “Uniform Resource Locators,” Internet Request for Comments (RFC) 1738, by T. Berners-Lee, L. Masinter, M. McCahill, University of Minnesota, December 1994. For further discussions of relative URL format and usage, see “Relative Uniform Resource Locators,” RFC 1808, by R. Fielding, University of California, Irvine, June 1995.
A hypertext link to an electronic document is specified by one of several HTML elements. One of the parameters of an HTML element for a hypertext link is the URL that serves as the identifier for the target of the link. An HTML document may have a base element defining an absolute URL that specifies the base URL for that document. If the document has no base element, then the absolute URL of the document is used as the base URL. The base element provides a base address for interpreting relative URLs when the document is read out of context.
For example, FIG. 7A shows text with a document URL 200, a base element 202, a hypertext link with an absolute URL 204, and a hypertext link with a relative URL 206, which is evaluated with respect to base element 202 to produce a resulting URL 208. As an additional example, FIG. 7B shows text with a document URL 210, no base element, a hypertext link with an absolute URL 212, and a hypertext link with a relative URL 214, which is evaluated with respect to document URL 210 to produce a resulting URL 216.
A site at which documents are made available to network users is called a “Web site” and must run a “Web server” program to provide access to the documents. A Web server program is a computer program that allows a computer on the network to make documents available to the rest of the WWW or to a private network. The documents are often hypertext documents written in the HTML language, but may be other types of document that include other types of objects as well, such as images, audio, and/or video data. The information that is managed by the Web server includes hypertext documents that are stored on the server or are dynamically generated by scripts on the Web server. Several Web server software packages exist, such as the Conseil Europeen pour la Recherche Nucleaire (CERN, the European Laboratory for Particle Physics) server or the National Center for Supercomputing Applications (NCSA) server. Web servers have been implemented for several different platforms, including the Sun SPARC II™ workstation running the Unix operating system, and personal computers with the Intel PENTIUM™ processor running the Microsoft MS-DOS™ operating system and the Microsoft WINDOWS™ graphic user interface operating environment.
Web servers also use a standard interface for running external programs, such as the Common Gateway Interface (CGI) or ISAPI. CGI is described in more detail in How to Set Up and Maintain a Web Site, by Lincoln D. Stein, Addison-Wesley, August 1995. A gateway is a program that handles incoming information requests and returns the appropriate document or generates a document dynamically. For example, a gateway might receive queries, look up the answer in a database to provide a response, and translate the response into a page of HTML so that the server can send the response to the client. A gateway program may be written in a language such as “C” or in a scripting language such as Perl or Tcl or one of the Unix operating system shell languages. The CGI standard specifies how the script or application receives input and parameters, and specifies how output should be formatted and returned to the server.
For security reasons, a Web server machine may limit access to files. To control access to files on the Web server, the Web server program running on the server machine may provide an extra layer of security above and beyond the normal file system and login security procedures of the operating system on the server machine. The Web server program may add further security rules such as: (a) optionally requiring input of a user name and password, completely independent of the normal user name and passwords that the operating system may maintain on user accounts; (b) allowing groups of users to be identified for security purposes, independent of any user group definitions defined in the security components of the operating system; (c) access control for each document object such that only specified users (with optional passwords) or groups of users are allowed access to an object, or so that access is only allowed for clients at specific network addresses, or some combination of these rules; (d) allowing access to the document objects only through a specified subset of the possible HTTP methods; and (e) allowing some document objects to be marked as HTML documents, others to be marked as executable scripts that will generate HTML documents, and others to be marked as other types of objects such as images. Access to the on-line service document objects via a network file system would not conform to the security features of the Web server program and would provide a way to access documents outside of the security provided by the Web server. The Web server program also typically maps document object names that are known to the client to file names on the server file system. This mapping may be arbitrarily complex, and any author or program that tries to access documents on the Web server directly would need to understand this name mapping.
A user (typically using a machine other than the machine used by the Web server) who wishes to access documents available on the network at a Web site must run a Web browser program. The combination of the Web server and Web browser communicating using an HTTP protocol over a computer network is referred to herein as a “web architecture.” The Web browser program allows the user to retrieve and display documents from Web servers. Some of the popular Web browser programs are: NAVIGATOR™ browser from NetScape Communications Corp., of Mountain View, Calif.; MOSAIC™ browser from the National Center for Supercomputing Applications (NCSA); WNWEB™ browser, from Microelectronics and Computer Technology Corp. of Austin, Tex.; and Internet Explorer from Microsoft Corporation of Redmond, Wash. Web browsers have been developed to run on different platforms, including personal computers with the Intel Corporation PENTIUM™ processor running Microsoft Corporation's MS-DOS™ operating system and Microsoft Corporation's WINDOWS™ graphic user interface environment, and Apple Corporation's MACINTOSH™ personal computers, and other independent operating systems, such as Linux.
The Web server and the Web browser communicate using the Hypertext Transfer Protocol (HTTP) message protocol and the underlying transmission control protocol/Internet protocol (TCP/IP) data transport protocol of the Internet. HTTP is described in Hypertext Transfer Protocol—HTTP/1.0, by T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen, Internet Draft Document, Oct. 14, 1995. In HTTP, the Web browser establishes a connection to a Web server and sends an HTTP request message to the server. In response to an HTTP request message, the Web server checks for authorization, performs any requested action, and returns an HTTP response message containing an HTML document in accord with the requested action, or an error message. The returned HTML document may simply be a file stored on the Web server, or may be created dynamically using a script called in response to the HTTP request message. For instance, to retrieve a document, a Web browser may send an HTTP request message to the indicated Web server, requesting a document by reference to the URL of the document. The Web server then retrieves the document and returns it in an HTTP response message to the Web browser. If the document has hypertext links, then the user may again select one of those links to request that a new document reference by the selected link be retrieved and displayed.
As another example, a user may fill in a form requesting a database search. In response, the Web browser will send an HTTP request message to the Web server including the name of the database to be searched, the search parameters, and the URL of the search script. The Web server calls a search program, passing in the search parameters. The program examines the parameters and attempts to answer the query, perhaps by sending the query to a database interface. When the program receives the results of the query, it constructs an HTML document that is returned to the Web server, which then sends it to the Web browser in an HTTP response message.
Request messages in HTTP contain a “method name” indicating the type of action to be performed by the server, a URL indicating a target object (either document or script) on the Web server, and other control information. Response messages contain a status line, server information, and possible data content. The Multipurpose Internet Mail Extensions (MIME) specification defines a standardized protocol for describing the content of messages that are passed over a network. HTTP request and response messages use MIME header lines to indicate the format of the message. MIME is described in more detail in MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies, Internet RFC 1341, June 1992.
Internet users typically access web resources through one of three ways: (1) by directly entering (e.g., typing in) the URL for the resource, such as http://www.Microsoft.com; (2) through a reference in another document, such as a hyperlink; or (3) through a separate storage of the link's URL, such as a listing under a “Favorites” (or Bookmarked) menu item in a browser, a folder view of the browser's history, or the results displayed by an Internet search engine. These methods all work equally well as long as the URL for the linked document or site doesn't change. Unfortunately, changes in web pages and sites are very common, and URL's for sites and documents are constantly being changed. When a hyperlink's URL no longer points to its (previously) associated resource (e.g., a web page), the hyperlink is said to be “broken.” In such instances, the URL entry provided by any of the foregoing methods will not locate the resource it was previously mapped to unless there is some provision for forwarding the user to the new URL. For instance, the author of a site can associate some HTML code with the previous URL that automatically forwards a user traversing the link to the new URL. Unfortunately, there is no facility built into the Internet's URL referential addressing scheme that automatically remaps the locations of web resources. As a result, it is very common for users to receive a “Document/Page not Found” error when a web page has been moved, and the prior URL is no longer valid.
Conventional web authoring tools only provide a partial solution to the foregoing resource relocation problem. For example, Microsoft Corporation's FRONTPAGE™ maintains lists of links within a currently authored web site, and ensures that when pages are moved, the links to the moved pages that are located in other pages within the same web site are updated. For instance, if a FRONTPAGE™ user is authoring a web site and moves one of the documents, all of the hyperlinks within the site are automatically updated to map to the page's new location. However, this does not address the other commonly encountered problems concerning broken hyperlinks discussed above, such as when the web page is linked through an external reference (i.e., external relative to the web site). In particular, it would be advantageous to provide a scheme that automatically updates broken URL references so that the resources previously associated with the broken URLs can be more easily located.